The present invention relates to light quantity adjusting devices provided in still cameras, video cameras, or other image pickup apparatuses to drivingly open and close blades such as shutter blades or restricting blades to adjust the quantity of photographing light. In particular, the present invention relates to improvements in operation mechanisms for the blades such that the operation mechanisms can smoothly operate the blades.
In general, the light quantity adjusting device adjusts the quantity of light for a shutter or a diaphragm by using an electromagnetic driver to open and close a blade located on a photographing optical axis of a camera or the like. The electromagnetic driver magnetically rotates a rotor rotatively provided in a stator to open and close the blade.
There has been a strong demand for a reduction of size of the electromagnetic driving light quantity adjusting device. For example, a camera incorporated into a small-sized instrument such as cellular phone, light quantity adjusting device such as shutter blade or restricting blade are required to have a drastically reduced size and reduced power consumption. Thus, a driving mechanism that opens and closes the blade is small and light. In particular, the diameters of a magnet and an exciting coil have been significantly reduced. For example, the diameter of a yoke has been reduced to about 5 to 6 mm.
The reduced size of the driver reduces the mass of a driving rotating portion, possibly causing, for example, a hunting phenomenon. In this phenomenon, when the blade is topped by abutting against a stopper, the entire driving system vibrates. The hunting phenomenon of the driving system varies between a light quantity adjusting blade composed of one blade and a light quantity adjusting blade composed of two blades that are opened and closed in the opposite directions. If the light quantity adjusting blade is composed of one blade, all of the impact resulting from striking of the blade against the stopper acts on a driving arm provided on a driving rotating shaft. This causes damped vibration in the arm and blade. On the other hand, if the light quantity adjusting blade has two blades, an impact on the blade acts on the right and left arms provided on the driving rotating shaft in the opposite directions. This prevents a possible significant phenomenon.
In connection with opening and closing of the blade by the electromagnetic driver, for example, Japanese Patent Laid-Open No. 11-194384 proposes that the blade be prevented from being moved at a fully open position or a closed position by the jolting driving system. Japanese Patent Laid-Open No. 11-194384 discloses a mechanism in which a transmission pin is provided for a rack gear coupled to a stepping motor and is engaged with a slit-like cam groove formed in the blade so that the rack gear slides to open and close the blade via the cam groove in a direction orthogonal to that in which the rack gear slides. The inclination of the cam groove is varied so that in a certain groove area, the blade is not moved in the opening or closing direction in spite of movement of the transmission pin.
As described above, to deal with the impact caused in case the blade is opened and closed by the electromagnetic driver and abuts against the stopper or the like at the fully open and fully closed positions, two blades are combined together so as to open and close in the opposite directions and the opposite ends of the driving arms provided on the driving rotating shaft are engaged with the slit-like cam groove formed in the blades. The opposite ends of the arms cause the impact acting on the blades in the opposite directions relative to the driving rotating shaft to prevent hunting from occurring in the blade and driving system as a result of vibration. Also in this case, a hunting phenomenon may occur in which the blade is vibrated by jolting of a driving system such as the engaging portion of the cam groove. However, the blade, including a driving system such as a motor, does not vibrate violently.
On the other hand, Japanese Patent Laid-Open No. 11-194384, described above, discloses that the cam groove formed in the blade and the driving pin are appropriately shaped so that when the blade is stopped, no driving force is transmitted from the transmission pin to the cam groove. However, Japanese Patent Laid-Open No. 11-194384 does not make any proposal for prevention of a hunting phenomenon that may occur when the blade is stopped.
However, if the blade is composed of one blade, the driving arm, attached to the driving rotating shaft, needs to extend from the rotating shaft toward the blade like a cantilever. This biases the arm of the rotating shaft toward one side. In this case, the reduced mass of the rotor including the rotating shaft may cause the rotating shaft to vibrate rotatively as a result of an impact acting on the driving arm when the blade is stopped. When combined with the electromagnetic rotation of the rotor, the rotative vibration is disadvantageously intensified, resulting in a long time required for stabilization. Accordingly, the blade composed of one blade causes mechanical and magnetic unbalance in the driving system drivingly opening and closing the blade. Unfortunately, an impact on the blade intensifies the vibration of the driving system, making the hunting phenomenon of the blade significant.
Moreover, owing to the magnetic unbalance in the driving system, when a stepping motor is used as a driver, a change of rotor step angle from that at the blade stop position causes a magnetic attractive force to act on the rotor, intensifying vibration. Thus, the reduced mass of the driving mechanism may result in more violent vibration and a more significant hunting phenomenon. Therefore, a reduction in size and weight of the driving mechanism may disadvantageously affects the operation of opening and closing the blade.
An object of the present invention is to provide a light quantity adjusting device that prevents possible violent vibration resulting from an external force such as an impact acting on a blade even when a driving mechanism for opening and closing the blade has a reduced size and a reduced weight, so that the light quantity adjusting device enables the blade to be controllably opened and closed stably.
Another object of the present invention is to provide a light quantity adjusting device that can accurately and stably adjust the quantity of light without causing a hunting phenomenon even when a driving mechanism for a single blade used to adjust the photographing light quantity has a reduced size and a reduced weight.
Further objects and advantages of the invention will be apparent from the following description of the invention.